32 MOTIVE POWER OF HEAT. 



the same if the volume is further reduced from a 

 half litre to a quarter litre, from a quarter litre to 

 an eighth, and so on. 



If, instead of compressing the air, we carry it 

 successively to two litres, four litres, eight litres, 

 etc., it will be necessary to supply to it always equal 

 quantities of heat in order to maintain a constant 

 temperature. 



This readily accounts for the high temperature 

 attained by air when rapidly compressed. We 

 know that this temperature inflames tinder and 

 even makes air luminous. If, for a moment, we 

 suppose the specific heat of air to be constant, in 

 spite of the changes of volume and temperature, 

 the temperature will increase in arithmetical pro- 

 gression for reduction of volume in geometrical 

 progression. 



Starting from this datum, and admitting that 

 one degree of elevation in the temperature cor- 

 responds to a compression of T -\- , we shall readily 

 come to the conclusion that air reduced to -fa of 

 its primitive volume should rise in temperature 

 about 300 degrees, which is sufficient to inflame 

 tinder.* 



* When the volume is reduced TT ^, that is, when it 

 becomes yyf of what it was at first, the temperature rises 

 one degree. Another reduction of TT ^ carries the volume 



